This study was carried out to investigate the effect of in-ovo injection of leptin hormone on some growth and reproductive parameters in Japanese quail. A total number of 1200 Japanese quail eggs were incubated at 37.5°C and 50-60 % relative humidity, with automatic turning every two hours till the 15th day of incubation. On the 5th day of incubation (day of injection), the eggs were randomly divided into four equal groups: group 1, eggs were kept without treatment as a control group (C); group 2, eggs were injected with 50.0μl normal saline as a control for leptin solvent (N.S); group 3, eggs were injected with 50.0μl normal saline containing 0.1μg leptin (0.1 L) and group 4, eggs were injected with 50.0μl normal saline containing 1.0μg leptin (1L). After hatching, hatchability percentages and hatching weights were recorded. Twenty birds (ten males and ten females) from each group were identified and weighed weekly for successive 8 weeks to evaluate the live body weights. From the 4th week of age 10 birds (5 males and 5 females) from each group were killed and plasma samples were collected to determine plasma concentrations of testosterone and estradiol E2. The results of this study revealed that in-ovo administration of leptin in Japanese quail resulted in nonsignificant increases in hatchability percentages but it significantly increased (P≤0.05) the hatching weights. Also, this study showed that the live body weights of both male and female Japanese quail increased after in-ovo leptin administration. There were significant increases (P≤0.05) in the concentrations of testosterone and estradiol E2 in leptin treated groups, compared with control and normal saline treated groups.